1. Field of the Invention
The present invention relates to a method of capturing satellite radio waves by using an oscillator of a low accuracy in a GPS receiver.
2. Description of the Related Background Art
A GPS (Global Positioning System) is a position measuring system using artificial satellites. According to the GPS, six orbits which circulate the earth are set in the sky of an altitude of about 20200 km, a plurality of artificial satellites are put onto the orbits, and GPS signals including navigation data for measuring the position are transmitted to the earth from the satellite by a spread spectrum system. The radio waves of three or four satellites which are necessary to measure the position are received by a GPS Receiver on the ground (or sea or air), thereby enabling position information such as latitude, altitude, and the like at the signal received position which are necessary to be processed in a real-time manner by using the navigation data included in the GPS signal from each of the satellites.
The GPS signal which is released to the private citizen is a signal called a C/A code and is transmitted at a frequency of 1575.42 MHz. The capture the satellite radio wave, therefore, it is sufficient for the receiver to be in a standby mode at the frequency of 1575.42 MHz, in principle. Since the GPS satellite is not a geostationary satellite, however, the received frequency is fluctuated due to a Doppler effect and, further, an oscillating frequency of the oscillator of the GPS receiver is also fluctuated due to an offset.
To actually capture the satellite radio wave, therefore, a search center frequency of the satellite radio wave is set by deviating the frequency by a frequency corresponding to only the deviation amount of the received frequency due to the Doppler effect (hereinafter, such deviation amount is referred to as a "Doppler frequency deviation") and an offset amount of the oscillator. The search center frequency is set to a start point and the satellite is searched while gradually widening the search band width. At a time point when the oscillator is locked to the satellite radio wave, the capture of the relevant satellite radio wave is completed. By repeating such a capturing operation, the respective radio waves from satellites of the number which are necessary for position measurement are captured.
In the capturing operation of the satellite, as the search center frequency becomes closer to the actual received frequency of the satellite radio wave, the time which is required to capture the satellite radio wave decreases more and more. Main factors to set the search center frequency are the Doppler frequency deviation and the offset value of the oscillator. The Doppler frequency deviation is a phenomenon which inevitably occurs due to the earth circulating motion of the satellite and its value (up to a maximum of about .+-.6 kHz) can be previously obtained from the orbit data of the satellite and the schematic present location data. To set the search center frequency to a value which is as close to the actual received frequency as possible, it is desirable to reduce an error of the offset value of the oscillator of the GPS receiver to be as small as possible.
For this purpose, the conventional GPS receiver uses a crystal oscillator (hereinafter, referred to as "TCXO") with an accuracy compensation of an extreme high accuracy as an oscillator. According to the TCXO, by compensating the temperature characteristic of a quartz resonator by an electric circuit, an extremely high accuracy of a range of about .+-.2 to .+-.5 [ppm] (about .+-.3 to .+-.8 [kHz] as a frequency) is given in a wide temperature range of -40.degree. to +85.degree. C. An accuracy of the oscillator which is used in an ordinary communicating apparatus generally lies with a range of about .+-.50 to .+-.100 [ppm], preferably, about .+-.20 [ppm].
In the conventional GPS receiver, since the TCXO of a high accuracy is used, the value obtained at the time of the preceding position measurement is stored (backed up) into a memory in the receiver as an offset value of the oscillator of the GPS receiver, and the stored offset value is read out at the time to start the next capturing operation of the satellite radio wave and is used. When the user tries to use an oscillator of a low accuracy, since a change in the offset value is large, the backup value obtained at the preceding position measurement cannot be used as it is. To capture the satellite radio wave, it is necessary to search a desired satellite over a fairly wide frequency range, so there is a problem in that it takes a long time to capture the satellite radio wave.
Since the conventional GPS receiver uses the TCXO of a high accuracy, the aging change of the offset value is ignored and even after the first satellite radio wave was captured, the searching operation of another satellite is continued without updating the offset value. In the case where the initial set offset value has largely been changed due to some causes, therefore, even when the TCXO of the high accuracy is used, the satellite radio wave cannot be captured so long as the satellite radio frequency is not searched over a wide frequency range. It takes a fairly long time to capture radio waves from a plurality of satellites which are necessary for position measurement.
The TCXO used in the conventional GPS receiver, further, is very expensive as compared with a crystal oscillator of a low accuracy which is used in an ordinary communicating apparatus. The use of such an expensive TCXO exerts a large influence on the manufacturing costs of the GPS receiver.